Hitherto, very little literature has been published on the surface modification of polyisocyanates which are solid at room temperature.
German Auslegeschrift No. 25 57 407 describes a process in which a solution of a polyisocyanate in a low-boiling solvent is sprayed into a reaction zone with a gaseous di- and/or polyamine to form hollow beads of polyurethane polyurea through the reaction of the polyisocyanate with the amine and by evaporation of the solvent. The reaction is generally controlled in such a way that the isocyanate groups react off completely with the amine and any other NCO-reactive components added.
U.S. Pat. No. 4,070,346 describes the production of finely powdered particles ranging from 1 to 100 .mu.m in diameter by introducing liquid or molten polyisocyanate or NCO-terminated prepolymer droplets into a vapor chamber with volatile di- or polyamines. The particles after rapid removal from the reactor still contain free isocyanate groups in their interior beneath the solid polyurea skin. Particles having a residual NCO-content of from about 50 to 60% in their interior can be produced according to this process. Mixtures of polyisocyanates or isocyanate-terminated prepolymers with polyols (for example 1,4-butane diol) may also be sprayed in the diamine atmosphere and reacted to form particles.
U.S. Pat. No. 3,409,461 describes the coating of polyisocyanates with a protective substance, preferably a polymer. To this end, the isocyanate is dispersed in a solution of the polymer in a low-boiling solvent which has very little dissolving effect on the isocyanate and the resulting solution is spray-dried. A finely ground (particle size 1 to 10 .mu.m) naphthylene-1,5-diisocyanate is preferably spray-dried with a 1 to 2.5% solution of polystyrene, polyvinyl butylether, chlorinated rubber and the like in tetrachloromethane. Free-flowing powders having particle sizes from about 1 to 50 .mu.m are obtained. These powders are suitable for improving the adhesion of polyester products (woven fabrics, fibers, films) to rubber elastomers. In this process for coating isocyanates with polymers from solution, considerable quantities of toxic solvents may have to be used and removed. One particular disadvantage of the process lies in the high percentage of coating material (from 9 to 91% by weight: 50% by weight in the Examples) in the total weight of the coated isocyanate. As a result of this, an excessive proportion of troublesome foreign substance would have to be introduced in the production of high-quality polyurethanes.
U.S. Pat. No. 3,963,680 describes a further development of the encapsulation process described in U.S. Pat. No. 3,860,565 in which hardenable isocyanate mixtures are produced by the microencapsulation of up to 50% by weight of liquid trimerization catalysts, which contain from 1 to 25% by weight (based on catalyst) of primary or secondary organic polyamines, to form polyurea capsule walls. The catalysts are only activated in the isocyanates at elevated temperatures through destruction of the capsule walls. Further literature is cited in U.S. Pat. No. 3 963,680, particular reference being made to the work by Gutcho entitled "Capsule Technology and Microencapsulation", Noyes Data Corporation, Park Ridge, N.J./USA (1972).
German Offenlegungsschrift No. 1,570,548 describes a relatively long-lasting one-component system consisting of a mixture of (i) 1 mole of a polyester, polyether or polythioether, (ii) at least 1.5 moles of a solid isocyanate containing uret dione groups and having a melting point of 100.degree. C. or more, and (iii) at least 0.3 mole of a solid chain-extending agent containing OH-- and/or NH.sub.2 -groups and having a melting point of 80.degree. C. or more. At least 80% of the solid constituents of the mixture are required to have a particle size of 30 .mu.m or less. The shelf life of this one-component system amounts to between a few days and a few weeks at room temperature, but only to a few hours at 50.degree. C. One disadvantage of this known process lies in the fact that at least two of the three reactants have to be present in solid form to guarantee the requisite stability in storage. The effect of this is that the mixtures obtained generally have very high viscosities, and their viscosity continues to increase slowly because none of the compounds has been adequately modified in its reactivity. The reaction of diols at the surface of the solid diisocyanate particles, which is reflected in the steady increase in viscosity, takes place without control and too slowly in practice and does not sufficiently retard the reactivity of the polyisocyanates to the point where the system is self-stabilizing. In addition, when the mixture is hardened, inhomogeneities are inevitable in the fully heated product due to the high percentages of solid constituents. Processing of the highly viscous to solid mixtures is also more difficult because, in contrast to liquid mixtures, they first have to be brought into a formable condition either by increasing temperature or by applying pressure.
When high-melting polyisocyanates are mixed with relatively high molecular weight and low molecular weight polyols, a constant and relatively rapid further reaction takes place with a marked increase in viscosity. In other words, the surface reaction on the solid polyisocyanate particles does not form a coating around the polyisocyanate which is sufficient for retarding the reactivity.
British Pat. No. 1,134,285 describes a process for the production of dimeric diisocyanates in an aqueous reaction medium. According to this reference, dimeric diisocyanates produced in this way (for example dimeric tolylene diisocyanate), do not react with polyfunctional compounds containing reactive hydrogen atoms at room temperature, although mixtures with polyols, for example, can be thermally crosslinked to form polyurethanes. Stability may possibly be brought about by a slow surface reaction of isocyanates with water to form polyureas. According to this reference, primary or secondary diamines (for example, ethylene diamine) or polyamines (for example, triethylene tetramine) may also be used in the aqueous suspension of the dimeric diisocyanate, preferably in quantities of from 100 to 60% of the NCO-groups present. Crosslinking with dimeric diisocyanates produced in this way is generally brought about by splitting of the uret dione ring at high temperatures, for example in the range from 150.degree. to 200.degree. C.
Finally, German Auslegeschrift No. 3,112,054 (which corresponds to U.S. Pat. No. 4,400,497) describes a process for the production of storable, thermally hardening mixtures of polyisocyanate and polyol. In this case, the polyisocyanate is present in the form of solid particles in the polyol. The polyisocyanate particles are surface-deactivated to a level of from 0.5 to 20 equivalent percent of the total number of isocyanate groups present. Compounds containing reactive hydrogen atoms such as mono- or polyols, primary or secondary mono- or polyamines, and water are used for deactivation. The thermal crosslinking of these deactivated polyisocyanates, must be carried out at a relatively high temperature (165.degree. C.) because the reaction in question is not catalyzed.
There are references in the literature to a number of "blocked" polyisocyanates where the NCO-groups are completely reacted with H-reactive blocking agents, for example caprolactam and malonic esters. The splitting temperatures are generally very high, for example 150.degree. C. and higher. Amidines containing at least one isocyanate-reactive group (for example 2-phenyl imidazoline) have already been described as blocking agents (German Auslegeschriften Nos. 2,729,704 and 2,946,085). Although amidine-blocked polyisocyanates of this type show a slightly reduced splitting temperature (120.degree. C. or more), the entire blocking agent always must be split off. The same applies to the use of amidines as catalysts in the reaction of differently (for example phenol-) blocked polyisocyanates with relatively high molecular weight polyamines to form polyurethane ureas. Both the phenol blocking agent and the catalyst remaining in the system have to be removed (cf European Pat. No. 39,834).